N-myc is a member of a family of proto-oncogenes that encode related but distinct nuclear proteins. Myc genes are putative transcription factors that are thought to function in the regulation of gene expression in the context of cell growth and differentiation. However, the exact function of myc genes is unknown. Each myc gene is associated with a unique tissue-specific pattern of spontaneously arising tumors that partially reflects the expression pattern in normal development. The finding that a null mutation of N-myc when homozygous in the germline of the mouse is embryonic lethal suggests that N-myc plays a unique physiological role during development. N-myc is highly regulated during B lymphocyte development. Pre-B cells express both c-myc and N-myc. The expression of c-myc and N-myc mRNA is induced in normal pre-B cells by treatment with the pre-B cell specific growth factor IL-7; at this stage, regulation is mediated by releasing a block to transcriptional elongation. When cells mature to the B cell stage, transcriptional initiation of N-myc is down-regulated, while c-myc continues to be expressed. Together, these findings suggest that N-myc has a specific function in the early stages of B cell development. We hypothesize that N-myc is required for the expansion of precursor B cells either by promoting their growth, inhibiting their death, and/or preventing their terminal differentiation. The goal of these studies is to define precisely the function of N-myc during B lymphocyte development and how it relates to c-myc function. How N-myc may be involved in determination of other hematopoietic lineages will also be examined. The exact stages within B lymphocyte and T lymphocyte differentiation when N-myc is expressed will be determined. This expression will be correlated with expression of other stage-specific features within that lineage. The affect of a null N-myc mutation on development of committed and uncommitted progenitor cells will be determined. If a maturational block in B cell development is found, the differentiation stage at which it occurs will be defined and characterized. Transformed and nontransformed cell lines will be derived in order to determine how expression of other stage-specific genes are affected by lack of N-myc expression. The response of these cells to growth factor and mitogenic signals will be tested. A novel approach is presented for the derivation of immortalized cell lines that represent B cell differentiation stages and that retain full differentiative capacity is presented. Using "hit and run" gene targeting techniques, the sequences within each of the myc genes that determine their unique functions within lymphocyte differentiation will be determined. Finally, we will determine if N-myc may function during hematopoiesis by induction of apoptosis or modulation of c-myc-induced apoptosis. Besides providing insights into the molecular mechanisms underlying lymphocyte development, these studies may also yield revelations about myc gene function generally during embryogenesis and how transcription factors function in developmental processes. Furthermore, an understanding of how myc genes function in normal development may reveal how inappropriate expression leads to oncogenesis.